In the present specification, reference is made to the following publications illustrating conventional techniques.    [1] A. Dubietis, G. Jonusauskas, and A. Piskarskas, “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal,” Opt. Commun. 88(4-6), 437-440 (1992).    [2] I. N. Ross, P. Matousek, M. Towrie, A. J. Langley, and J. L. Collier, “The prospects for ultrashort pulse duration and ultrahigh intensity using optical parametric chirped pulse amplifiers,” Opt. Commun. 144, 125 (1997).    [3] I. N. Ross, P. Matousek, G. H. C. New, and K. Osvay, “Analysis and optimization of optical parametric chirped pulse amplification,” J. Opt. Soc. Am. B 19, 2945 (2002).    [4] S. Witte, R. T. Zinkstok, W. Hogervorst, and K. S. E. Eikema, “Numerical simulations for performance optimization of a few-cycle terawatt NOPCPA system,” Appl. Phys. B 87, 677 (2007).    [5] E. J. Grace, C. L. Tsangaris, and G. H. C. New, “Competing processes in optical parametric chirped pulse amplification,” Opt. Commun. 261, 225 (2006).    [6] P. Zhu, L. Qian, S. Xue, and Z. Lin, “Numerical studies of optical parametric chirped pulse amplification,” Optics and Laser Technology 35, 13 (2003).    [7] M. Guardalben, J. Keegan, L. Waxer, V. Bagnoud, I. Begishev, J. Puth, and J. Zuegel, “Design of a highly stable, high-conversion-efficiency, optical parametric chirped-pulse amplification system with good beam quality,” Opt. Express 11, 2511 (2003).    [8] F. Tavella, A. Marcinkevi{hacek over (c)}ius, and F. Krausz, “Investigation of the superfluorescence and signal amplification in an ultrabroadband multiterawatt optical parametric chirped pulse amplifier system,” New J. of Phys. 8, 219 (2006).    [9] J. A. Fueloep, Zs. Major, B. Horvath, F. Tavella, A. Baltu{hacek over (s)}ka and F. Krausz, “Shaping of picosecond pulses for pumping optical parametric amplification,” Appl. Phys. B 87, 79 (2007).    [10] S. K. Zhang, M. Fujita, M. Yamanaka, M. Nakatsuka, Y. Izawa, and C. Yamanaka, “Study of the stability of optical parametric amplification,” Opt. Commun. 184, 451 (2000).    [11] L. J. Waxer, V. Bagnoud, I. A. Begishev, M. J. Guardalben, J. Puth, and J. D. Zuegel, “High-conversion-efficiency optical parametric chirped-pulse amplification system using spatiotemporally shaped pump pulses”, Vol. 28, No. 14 OPTICS LETTERS 1245    [12] Pawe Wnuk, Yuriy Stepanenko, and Czesaw Radzewicz, “Multiterawatt chirped pulse optical parametric amplifier with a time-shear power amplification stage”, Vol. 17, No. 17/OPTICS EXPRESS 15264    [13] Thomas Metzger, Alexander Schwarz, Catherine Yuriko Teisset, Dirk Sutter, Alexander Killi, Reinhard Kienberger, and Ferenc Krauszl, “High-repetition-rate picosecond pump laser based on a Yb:YAG disk amplifier for optical parametric amplification” Vol. 34, 2123, OPTICS LETTERS (2009)    [14] M. Schulz, R. Riedel, A. Willner, T. Mans, C. Schnitzler, P. Russbueldt, J. Dolkemeyer, E. Seise, T. Gottschall, S. Hädrich, S. Duesterer, H. Schlarb, J. Feldhaus, J. Limpert, B. Faatz, A. Tünnermann, J. Rossbach, M. Drescher, and F. Tavella “Yb:YAG Innoslab amplifier: efficient high repetition rate subpicosecond pumping system for optical parametric chirped pulse amplification” 2456, Vol. 36, OPTICS LETTERS (2011)
Since the first demonstration of parametric amplification of chirped femtosecond pulses in β-barium borate (BBO) crystal by Dubietis et al. [1], optical parametric devices play a key role in many laser applications. They have proved versatile as widely tunable coherent sources, especially for short pulses since they can offer both high gain and high-gain bandwidth. Optical Parametric Chirped Pulse Amplifiers (OPCPAs) have become increasingly popular tools for the generation of high peak power pulses including pulses with TW and PW power levels in the visible to infrared range. Today there is much interest in the development of OPCPAs as a light source for few-cycle, high-intensity, near-to-mid-infrared pulse-driven applications such as high harmonic generation and attosecond techniques.
In OPCPA systems, a high-energy pump laser is coupled to a chirped, low-energy broadband seed field in an optically nonlinear crystal. If the seed pulse duration is stretched to match the pump pulse duration and ensure a proper temporal overlap, a reasonable amplification can be achieved. The efficiency is around 30%, including signal and idler.
In broad bandwidth OPCPA laser systems, the efficiency of the whole system depends on phase-matching temporal profile, temporal pump pulse profile, spatial pump beam profile, and pulse duration matching. The pump- and phase-mismatch temporal profiles cause spectral narrowing. In order to achieve the full phase-matching bandwidth of the amplifier, conventionally the seed pulse is kept short enough, relative to the pump pulse, that the pump intensity remains more-or-less constant across the duration of the seed. However, for very short seed pulses only a small fraction of the pump pulse is depleted and the efficiency decreases with increasing amplifier bandwidth.
On the other hand, the pump beam is Gaussian in time and space, and saturation is achieved at different lengths of the crystal for different positions in the beam, which means that some parts of the beam are not saturated while other are over-saturated. In order to achieve perfect saturation and close to complete conversion efficiency, conventionally an ideal square spatial and temporal pump pulse profile, a perfect phase-match temporal profile and pulse duration matching was needed.
FIGS. 8 to 10 schematically illustrate examples of the conventional OPCPA techniques using a single amplification stage (FIGS. 8, 9) or multiple amplification stages (FIG. 10). FIG. 8 (prior art) schematically illustrates the scheme of a conventional OPCPA system including a pump pulse laser 10′ and a seed pulse laser 20′. The pump pulse laser 10′ creates laser pulses e.g. having a pump pulse duration of 1 ps and a pump pulse energy of 10 mJ while the seed pulse laser creates laser pulses having a seed pulse duration of 0.7 ps and a seed pulse energy of 0.1 mJ. The pump and seed pulses 1′, 2′ are superimposed in the optically non-linear OPA crystal 30′, where an amplified signal pulse and an idle pulse are output. FIG. 9 (prior art) illustrates examples of an output spectrum of the signal pulses (FIG. 9A), a dependency of the output energy on the length of the OPA crystal (FIG. 9B) and a phase mismatching of the signal pulses (FIG. 9C) using one single amplification stage only. The maximum output obtained with the technique of FIG. 8 using the above examples is e.g. 1.8 mJ, wherein the efficiency is about 36% (signal+idle pulses).
According to FIG. 8 (prior art), since the pump beam is Gaussian in time and space, the gain of the OPCPA is highest in the peak point. The pump pulse depletes fastest in the peak point and saturates first. Afterwards, when the other parts are becoming saturated, the parts which were saturated are undergoing back conversion. In order to amplify the whole bandwidth of the seed, the seed pulse duration is shorter than the pump pulse duration. The part energy of pump pulse which has no overlap with seed is wasted.
A prerequisite for the development of OPCPA is a careful investigation into the simultaneous optimization of conversion efficiency, and signal bandwidth. Previous studies of OPCPA optimization have treated several aspects of this problem independently [2-10]. The two earlier works by Ross et al. [2, 3] discuss the trade-off between amplifier bandwidth and conversion efficiency that is determined by the signal pulse chirp. Publication [3] finds that amplifier conversion efficiency and efficiency-bandwidth product are maximized at approximately the same propagation length, and for Gaussian pump and seed pulses finds that a ratio of seed and pump pulse durations, of 0.57 optimizes the efficiency-bandwidth product. A recent work by Witte et al. [4] on a terawatt non-collinear OPCPA system focuses on the spectral shaping due to both the phase-matching conditions and the pump intensity profile, and finds a different optimum, 0.2-0.3. This discrepancy highlights the need for a systematic analysis of the optimal seed/pump duration ratio and of its dependence on OPCPA parameters, such as the total gain.
Until right now, there are two ways to increase the efficiency of OPCPA system, which are described as follows. Firstly, spatiotemporally shaped pump pulses can be used in order to optimize the OPCPA conversion efficiency by homogeneously depleting the pump in space and time [11]. In this method, a modified pump spatial and temporal profiles is created. A flat-top spatial profile and high order super-Gaussian temporal profile are often used in such method. By using a spatiotemporally shaped pump pulse to maximize the conversion efficiency of OPCPA, pump-to-signal conversion efficiency can reach 29%.
Secondly, a time-share power amplification stage can be used in order to increase conversion efficiency from a long pump pulse [12] as shown in FIG. 10 (prior art). With this technique, pump pulses (8 ns) from a pump pulse laser 10′ are superimposed with seed pulses (1 ns) from a seed pulse laser 20′ at three amplification stages 31′, 32′ and 33′. The light path of the seed pulses is adjusted so that the seed pulses are superimposed with different ranges of the pump pulses at each amplification stage. Thus, a stepwise increasing signal pulse is created as schematically illustrated in the lower part of FIG. 10. In other words, to achieve high energy conversion efficiency, each pass of a relatively short seed pulse through the nonlinear crystal is delayed with respect to a long pump pulse as shown in FIG. 10. After three amplification stages 31′, 32′ and 33′, a conversion efficiency of 34% was obtained for a 3 ns window of the 8 ns pump pulses. This corresponds to a totally efficiency of around 17%.